The present invention relates to a magneto-optic recording medium capable of erasing, recording, and reproducing information by means, for example, of a laser beam.
Magneto-optic recording media are been actively studied as a candidate for rewritable optical recording media, and rare earth-transition metal alloy thin films are widely used as a magnetic film thereof. Rare earth-transition metal alloy thin films however have insufficient magneto-optic effect (Kerr effect, for example) and playback signals obtained do not have sufficient carrier-to-noise (C/N) ratio.
As a measure to compensate for the insufficient magneto-optical effect to give rotation of the plane of the polarized light, use of a reflective film was suggested, as, for example, in Japanese Patent Application Kokai Publication No. 194,664/1986.
FIG. 2 shows a prior-art magneto-optical recording medium. As illustrated, it comprises a substrate 1, a dielectric film 21, a magnetic film (recording layer) 3 formed of a rare earth-transition metal alloy thin film, another dielectric film 22, and a reflective film 4. Part of the laser beam L incident on the magnetic film 3 is reflected at the surface of the magnetic film 3 and is subjected to Kerr rotation. The magnetic film 3 is sufficiently thin so that part of the laser beam L incident on the magnetic film 3 is transmitted through the magnetic film 3 and is reflected at the surface of the reflective film 4, and then is transmitted through the magnetic film 3 again. During the transmission, the laser beam L is subjected to Faraday rotation. As a result, the Kerr rotation given upon the reflection at the surface of the magnetic film 3 and the Faraday rotation given upon transmission through the magnetic film 3 are added to each other. In other words, apparent Kerr rotation is increased.
Moreover, by appropriately choosing the refraction index and the thickness of the dielectric film 22, the repeated reflection in the dielectric film 22 can be utilized to increase the apparent Kerr rotation further. The reflective film 4 must have a sufficient reflectivity, and is formed of Al, Au, Ag and the like.
But with the prior-art magneto-optical recording medium described above, although the apparent Kerr rotation is increased, because the dielectric film 22 acts as an anti-reflection film, the reflectivity is decreased, and because of the thermal conductivity of the dielectric film 3, the writing sensitivity is not sufficient. This is because the high thermal conductivity acts to rapidly diffuse heat generated by the local heating by irradiation with the laser beam for the purpose of recording information.
To eliminate this problem it was suggested to form the magnetic film 3 directly on the reflective film 4 of Al, Au, Ag or the like. But because of the high thermal conductivity of the reflective film 4, the writing sensitivity is lowered.